The present invention relates to a tool for breaking a shaft of a surgical pin or screw, e.g. a Schanz-type screw, in order to reduce the excess portion of the pin extending beyond a fixator in bone surgery.
External fixators are widely used in bone surgery in order to reduce the fracture of a bone and to maintain the bone fragments in a fixed relationship. Commonly, pins are inserted into the bone for connection to the fixator. Such pins and/or screws often are of an undefined or uniform length, making it desirable for their excess shaft portions projecting beyond an exposed surface to be removed. For this purpose, various pin cutter devices have been developed.
For instance, U.S. Pat. No. 6,085,425 relates to a surgical cutter having two cooperating jaws pivotally secured in between two holding plates by floating bolts, and having said jaws pivotally secured to two very long handles in order to build up the large leverage force necessary.
U.S. Pat. No. 5,513,434 relates to a pin cutter with a cutting wheel for forming an annular groove about the periphery of the pin. Therefore, the surgical pin is positioned between cylindrical rollers and the cutting wheel. A thumb wheel is turned onto external threads of an internal sleeve, causing the sleeve to be drawn further into the cutter housing for increased compression of the cutting wheel into the pin. A camming surface of the housing contacts externally flared walls of a pair of spaced legs on the distal end of the cutter. By the movement of the sleeve, the legs are biased inwardly, causing the cutting wheel to push further into the pin. Handles are used to rotate the cutter wheel to deepen the groove in the periphery of the pin. Thereby, the pin can be completely severed or, alternatively, snapped at the line of the peripheral groove formed by the cutter.
DE 198 53 917 A1 discloses a pin cutter incorporating shearing elements driven by a rotary piston unit which is operated by a control valve. A billet-shaped handle coaxial with the longitudinal axis of the housing is positioned on the top housing-cover. The top end of the operating valve is integrated in the handle. The bottom housing cover has a holder for the plug-in connection for a cutter-head. The handle has a hole containing the projecting pin as an extension of the rotor's hollow shaft, the bottom end of which has an inner hexagonal part forming a drive connection with the shearing-pin. With this construction, the user does not have to incur large manual forces but can activate the device by pressing an actuation button. It is difficult to include such a compressed air using device in a surgical setting because maintenance and sterilization issues.
The shearing tool of U.S. Pat. No. 6,928,900 allows a threaded shaft, such as that of a screw, to be cleanly broken along a line by twisting contiguous shaft portions against each other. The device comprises two aligned tubular, preferably telescopically engaged elements, each having an end portion for receiving a contiguous portion of a shaft and being locked against rotation relative to the shaft portion contained therein. In order to relatively twist the shaft portions, the tubular elements are rotated relative to each other. The rotating means comprises two arms extending laterally from the common axis of the elements. By bringing together and forcing back the arms repeatedly, the shaft is ultimately fractured. This device allows the shaft to be sheared at their interface, achieving a substantially planar surface at the fracture plane.